"From aloft they had seen a dead plain of sterile earth or sand stretching to meet the face of the white mountain which was grotesquely hewn into the likeness of a grinning skull. (...) But now that cliff presented a wholly different aspect. Conan's volcanic gaze narrowed and a tingle of supernatural awe prickled his nape. For the cliff, which from the air seemed to have been carven into the form of a skull, now appeared as the facade of a splendid, ornate palace." -- L. Sprague de Camp and Lin Carter: "Shadows in the Skull"
A city in the Land of No Return, a region far to the south of Zembabwei, beyond the Great Waterfall, surrounded by high mountains. It had bleak plateaus, scrubland, and a sandy plain.
Yanyoga was masked from observation by the Great Stone Skull, a geologic formation or mirage in the shape of a skull. Geysers and fumaroles in the land around it suggest volcanic activity.
Yellow-skinned aborigines of the region were enslaved by a remnant of the ancient serpent-race that ruled the world before the coming of men. Yanoga was said to be the last stronghold of the serpent-folk.
References: Shadows in the Skull.